Impacts of flow regulation on slackwaters in river channels

Flow regulation between irrigation storages and demand offtakes often results in prolonged elevated flows in the river channel during a period that would otherwise typically experience lower flows. This is a growing concern worldwide with negative implications for aquatic flora and fauna susceptible to changes in the hydraulic environment. Slackwaters, a hydraulic patch characterized by low velocities and shallow depths, are important habitat for zooplankton and have been associated with an order of magnitude more fish and shrimp than midchannel patches. Slackwaters are vulnerable to flow regulation and reductions in slackwater area have been shown to reduce abundances of juvenile and larval fish with poor swimming abilities; invertebrates dependent on slackwater refugia; and organic matter retention. It is therefore surprising that the relationship between flow regulation and slackwaters has not been explicitly quantified. In this study, we quantify changes to slackwaters (by surface area relative to inundated area) under the influence of flow regulation, specifically total area, patch area, and number of patches. Using two-dimensional hydraulic modeling for sites on the Broken River, southeastern Australia, we demonstrate that the area of slackwaters decreases with increasing discharge until inundation of higher-elevation bars and benches. We assess changes in slackwaters against three levels of increasing flow regulation intensity and find that increasing levels of regulation reduce slackwater area: With a heavily regulated scenario resulting in half the slackwater area and patch sizes 5 times smaller, compared with natural conditions. These findings provide the conceptual model and quantifiable relationships for testing population dynamics, identifying key hydrologic drivers of ecological decline and assisting river storage operators to manage flows during critical periods for biota.